1998 Mitsubishi montero the rpms fluctuate, idle or moving

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My montero is doing this right now. Alternators been replace 3x, battery is new, replaced the MAF sensor. Also changed the ECU. Not sure what else to do. These were all recommended by our mechanic and with each repair it works fine til the end of the street and it starts doing it again. It sort of thrusts forward at the same time the rpms fluctuate.